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From: Kaz Kylheku <643-408-1753@kylheku.com>
Newsgroups: comp.unix.shell
Subject: Re: (shellcheck) SC2103
Date: Wed, 5 Mar 2025 18:40:30 -0000 (UTC)
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On 2025-03-05, Kenny McCormack <gazelle@shell.xmission.com> wrote:
> All testing done with shellcheck version 0.10.0 and bash under Linux.
>
> Shellcheck says that you should replace code like:
>
>     cd somedir
>     do_something
>     cd ..	# (Or, cd -, which is almost, but not exactly the same thing)
>
> with
>
>     (
>     cd somedir
>     do_something
>     )

That obviously won't work if do_something has to set a variable
that is then visible to the rest of the script.

Forking a process just to preserve a current working directory
is wasteful; we wouldn't do that in a C program, where we might
open the current directory to be saved, and then fchdir back to it.

However, most of the actions in a shell script fork and exec
something anyway.

> The ostensible rationale is that it is shorter/easier to code, but the real
> rationale is that if the cd fails, putting it into a subshell "localizes"
> the damage.

  save_pwd=$(pwd)   # local save_pwd=$(pwd) in shells that have local

  if cd somedir ; then
    ...
    cd "$save_pwd"
  else
    ...
  fi

> but the more general pattern would be:
>
>     cd somewhere;...;cd -

cd - will break if any of the steps in between happen to to cd;
it is hostile toward maintenance of the script.

By the way, we should also try to exploit the capability of commands to
do their own chdir.

E.g:

  save_pwd=$(pwd)
  cd somewhere
  tar czf "$save_pwd"/foo.tar.gz .
  cd "$save_pwd"

becomes

  tar -C somewhere -czf foo.tar.gz .

tar will change to somewhere for the sake of finding the
files, and will resolve the . argument relative to that location,
but the foo.tar.gz file is created in the original directory
where it was invoked.

Another utility with -C <dir> is make.


-- 
TXR Programming Language: http://nongnu.org/txr
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